Gap junctions are responsible for intercellular propagation of electrical signals throughout the heart. Downregulation of connexin43 (Cx43), the predominant connexin expressed in mammalian ventricular tissue, is thought to underlie conduction abnormalities that may lead to lethal ventricular arrhythmias in diseased hearts. Expression of CA5 in normal ventricles is thought to be limited and of unknown physiologic relevance. We have obtained exciting new evidence of dramatic, focal increases in Cx45 expression in human hearts from patients with end-stage heart failure who are at high risk for developing malignant ventricular arrhythmias and sudden cardiac death. The present proposal will focus on a novel hypothesis that Cx45 is upregulated in a patchy, heterogeneous distribution in the diseased heart, which contributes to reduced intercellular coupling, discontinuous propagation and induction of arrhythmias. The specific aims are (1) to delineate the effects of Cx45 expression and Cx45/Cx43 stoichiometries on gap junction structure and distribution, (2) to delineate specific electrophysiologic parameters that are disrupted or induced by overexpression of CA5 and (3) to demonstrate that upregulation of CA5 in disease states is a major determinant of arrhythmogenesis. Structural and electrophysiologic responses to heart failure induced by pressure overload, and myocardial infarction induced by coronary artery occlusion, will be assessed in transgenic mice overexpressing Cx45 in the presence or absence of genetic deficiency in Cx43. Experimental methods used for assessing structural and electrical remodeling will include confocal immunohistochemistry, fight and electron microscopy, in vivo and in vitro electrophysiology studies and ambulatory ECG monitoring. These studies will provide novel fundamental insights into remodeling of coupling proteins that determine cellular mechanisms of arrhythmogenesis and the potential importance of intercellular coupling for development of new therapeutic strategies aimed at reducing the risk of sudden death associated with myocardial infarction and heart failure.